Dog owners have been warned to be extra vigilant after the appearance of mysterious chalk markings on properties following a spate of dognappings.

Expensive breeds of dog are being targeted by the crooks, who want them for their pedigree breeding or to use in sports such as hare coursing.

Homeowners have found strange chalk markings on several properties, which some have suggested were put there by criminals to denote an easy target.

The warning comes after unscrupulous thieves struck across Britain, snatching dogs which they believed might be valuable.

Last week the body of two-year-old cockapoo Willow, owned by Hayley Walsh and Ross Scotney, was discovered near a public footpath in Biggleswade, Bedfordshire, after she went missing on October 30.

Heartbroken Mr Scotney, 28, said: "We know five to ten people whose dogs have been stolen, some from gardens and others snatched while they were actually on a walk.

"There is a lot of activity in the area, particularly around the travellers' site." 

Police said that the spate of dog thefts in the area appeared to be focused on expensive breeds such as cocker spaniels and labradors, and cross-breeds like cockapoos, as well as gun dogs used on shoots.

They also steal 'running' dogs such as greyhounds, whippets, lurchers and Salukis to supply the black market trade for illegal hare coursing.

A petition has been started, calling on police to take stronger action, and within days it had been signed by more than 8,000 people.

Labradoodle breeder Annette Courtney, who signed the petition, said: "I know it can be difficult for the police but I do think they should investigate Willow's case because it was so bad.

"If someone took her and found out she'd been spayed, she'd be of no use to them and they'd just get rid of her."

Police fear that dognappers will beat dogs into submission before selling them on the black market.